1. Field of the Invention
This invention relates to an evaporative fuel-processing system for internal combustion engines for vehicles, which processes evaporative fuel generated in the fuel tank of the engine mounted in a vehicle.
2. Prior Art
An evaporative fuel-processing system for internal combustion engines for vehicles is conventionally known, for example, from Japanese Provisional Patent Publication (Kokai) No. 1-125553. FIG. 1 schematically shows the whole arrangement of the proposed conventional evaporative fuel-processing system, which comprises a canister a which is formed integrally with check valves b and c, a diaphragm value h, and an evaporative fuel-introducing port e. The check valves b and c are connected to an air cleaner k and the exterior of an engine room via conduits m and n, respectively. When the engine is in stoppage, the check valve c is opened by positive pressure within the canister, and evaporative fuel from a fuel tank f is guided via a charging passage i and the evaporative fuel-guiding port e into the canister a to be adsorbed by an adsorbent g. During traveling of the vehicle in which the engine is installed, the diaphragm value h and the check valve b are opened by negative pressure generated within an intake pipe of the engine, and evaporative fuel which has been collected by the adsorbent g is purged to the engine by hot air blown from the air cleaner k into the canister a via the check valve b. When evaporative fuel is generated in too large an amount to be collected by the canister a, or when pressure within the charging passage i is high, evaporative fuel within the canister a is discharged via the check valve c to the exterior of the engine room l.
However, the above conventional evaporative fuel-processing system has the following inconveniences.
FIG. 2 shows flow characteristics of the check valves. The check valves b and c have valve opening pressure P thereof set within a range of the order of a few mmHg's so as to minimize the intake pressure loss. As is apparent from the figure, the flow rate Q increases sharply with a pressure increase above the valve opening pressure by several mmHg's. FIG. 3 shows fluctuations in pressure within the conduit m caused by pulsations of intake air passing through the air cleaner k. The amplitude of the pressure fluctuations reaches as high as 17 to 20 mmHg when the engine is in a condition where the engine rotational speed is 4000 rpm and load on the engine is high with the throttle valve fully open, provided that the atmospheric pressure assumes a standard value. Thus, the amplitude of pressure fluctuations caused by pulsations of intake air passing through the air cleaner k is much larger than the valve opening pressure P of the check valves b and c, and consequently the check valve b can be made unstable in operation, resulting in leakage of evaporative fuel or air therethrough. Further, the above pressure fluctuations due to pulsations of intake air passing through the air cleaner k causes vibrations of valve seats of the check valves to incur wear thereof, resulting in degraded durability of the check valves.
Besides, if the conduit n is arranged such that its open end, which serves as an air-discharging port, terminates in a hollow portion of a chassis frame of the vehicle outside the engine room l, a smell of gasoline can used as the fuel fill the interior of the vehicle compartment when evaporative fuel is discharged from the canister a.
Next, another arrangement of the system will be considered wherein the check valves b and c as in the arrangement of the system in FIG. 1 are exchanged with each other. FIG. 4 schematically shows the whole arrangement of an evaporative fuel-processing system for internal combustion engines, which is distinguished from the arrangement of the FIG. 1 system only in that the check valves b and c are replaced with each other. With this arrangement, air is introduced from the exterior of the engine room l into the canister a and the air cleaner k serves as an air discharge port. This arrangement can prevent generation of a smell of gasoline from filling the vehicle compartment, which, however, cannot eliminate the inconvenience that the operation of the check valve c can be unstable due to fluctuations in the pressure within the conduit m due to pulsations of intake air passing through the air cleaner k. Rather, during stoppage of the engine, evaporative fuel is stored in the air cleaner k, and an uncontrolled amount of fuel will be drawn into the engine when it again operates, whereby the air-fuel ratio of a mixture supplied to the engine becomes overrich. As a result, further inconveniences, can be incurred such as starting failure of the engine, so-called hesitation, i.e. inability of the engine to increase its rotational speed when the accelerator is stepped on, and engine stalling.